Fiber reinforced epoxy composites have found widespread utility as structural materials. They have the advantage of having both high strength and high modulus to weight ratios, making them particularly useful in aeronautical and aerospace applications. A major drawback to these high modulus materials is their inherent brittleness which reslts in low impact strength.
It has been recognized that epoxy material can be modified by incorporating various reactive or inert material into their cure mixture and that by these modifications, the properties of the cured product can be varied. For example, U.S. Pat. No. 4,007,079 issued on Feb. 8, 1977 to Turely et al. shows that adding certain chlorine-containing polyols to an epoxy resin can reduce the resin's flamability. U.S. Pat. No. 2,833,681 issued on May 6, 1958 to Nelson et al. shows that using a fluorine-containing polycarboxylic anhydride as the cure agent in epoxy systems gives improved high temperature stability and properties. U.S. Pat. No. 3,879,490 issued Apr. 22, 1975 to Villa shows that an epoxy system is rendered inherently mold-releasable when it contains a reaction product of a hydroxyl or carboxyl-terminated polymer containing pendant vinyl or alkyl groups with perfluoroalkyl iodides. This compounding is also reflected in the toughening of epoxies by adding various rubber-like materials, most commonly carboxyl-terminated butadiene-acrylonitrile copolymers. This is discussed at Riew, C.K. et al. "Rubber Toughened Thermosets" in ACS Advances in Chemistry Series--Toughness and Bitterness of Plastics, Vol 27, p 330 which further describes a loss of modulus (stiffness) and a drop in ultimate use temperature.
Enhancement of the energy absorbing properties of thermoplastic and thermosetting resin systems such as the epoxies by adding rubber-like materials is recognized to depend upon the degree of rubber-resin compatability and upon the extent of dispersion of the rubber phase and control of its particle size. Bragaw, C. G., "The Theory of Rubber Toughening of Brittle Polymers," ACS Advances in Chemistry Series, "Multicomponent Polymer Systems", Vol. 176, points out at page 100 that in order to obtain a discrete rubber phase there must be a degree of incompatability between rubber and resin. This same article teaches at page 91 that, to insure efficient energy transfer, there must be molecular interaction between the two phases. The present invention serves to improve epoxy resin toughness without compromising the epoxy's high modulus, high strength and good thermal stability.